University of Vermont Extension
Department of Plant and Soil Science

Summer News
ArticleAPPLE PESTS AND DISEASES

Dr. Leonard Perry, Extension
Professor
University of Vermont

There
are 3 main insect pests you may find on apple trees in our region,
and 4 main
diseases. Knowing what to look for, and
how to control these at early stages or even to prevent them, will
make your
growing easier, more productive, and with less use of chemicals.

The
four main apple diseases, common in many areas of the country, are
apple scab,
cedar apple rust, powdery mildew, and fire blight. Hopefully you’ve
chosen cultivars (cultivated
varieties) resistant to some or all of these, such as ‘Freedom’ or
‘Liberty’. Other cultivars may tolerate
these diseases, so you still won’t have to spray. If looking to buy
some trees from your local
nursery to plant, check for their disease resistance. Other diseases
you may
see on apples are black rot and bitter pit.
More cultivars and their disease resistance can be found online
(homefruitgrowing.info).

Apple
scab is the most common disease. This
fungus causes olive-colored and velvety splotches on leaves, and
dark unsightly
blotches on fruits. Fruits infected
early in the season may fall off, and those infected later often
aren’t fit to
eat. This is more a problem in rainy and
wet seasons. Other than dormant oil
sprays in early spring, and fungicide sprays beginning in spring,
keeping
fallen leaves and fruit picked up in fall will help prevent this
disease.

There
are several rust diseases that attack leaves and fruit of apple
trees, with
cedar apple being the worst. As its name
indicates, it makes rusty spots on leaves, and attacks cedars too
(and, in fact,
needs them to complete its life cycle).
Remove cedars and junipers from within a few hundred feet (if
possible)
and you may eliminate this disease.

Powdery
mildew is descriptive of the symptoms you’ll see on leaves,
particularly in
warm and humid seasons. In addition to
good pruning to allow air circulation, there are several fungicides
for it
including some organic ones.

Fire
blight too is a descriptive name, its injury resembling burned tips
of
branches. Prune such injury out at the
first signs, disinfecting your pruning tools periodically with a
dilute
solution of bleach or household disinfectant (such as Lysol). This
disease is from a bacterium, so won’t
respond to fungicides, and is worse in some years than others.

Unfortunately,
there is isn’t the same cultivar resistance to insects on apples as
there are
to diseases. The main insects to watch
for are apple maggots, codling moths, and plum curculios (the latter
is a
problem in eastern states). Other pests
of apples you may see include aphids, spider mites, sawflies, and
San Jose
scale.

Apple
maggots may be known as “railroad worms” as they resemble small
worms that
railroad through the fruit leaving tunnels.
Small insects that look like houseflies lay eggs in growing fruit.
These hatch into the larvae, which live in
dropped fruit in fall and in the ground overwinter. So removing
such fruit in fall helps in
control, as do maggot traps. The latter
are red-shaped spheres with a sticky substance.
Another control is to place plastic sandwich bags over developing
fruit
after you’ve thinned them in early summer.
There are sprays, including the organic kaolin clay, which you can
use
in early and mid-summer if the problem is severe.

Another
“worm” you may find in apples is the larvae of the codling moth.
Look for holes in fruit with white to gray
grubs inside. Since the moth lays its
eggs in the flower at bloom time, the best control is to spray right
after
bloom, after the petals have fallen and the bees are gone. Special
traps can be put in the orchard to
lure the moths. Once the fruit start
forming, thinning them will help prevent the larvae moving among
fruit to
non-infected ones.

The
plum curculio, as its name indicates, also attacks plums as well as
most other
tree fruits east of the Rocky Mountains. The weevils leave a
characteristic
crescent-shaped scar on fruits as they’re forming, where they lay
eggs. This either causes fruits to drop, to become
infected with brown rot, or to be knobby and gnarled. Control these
with sprays after petals fall. Regularly pick up fallen fruit which
may
contain larvae, and discard in a trash can.
If not severe, birds and predators may keep insect populations
tolerable.

More on
all these and their controls can be found online from Cornell
University (www.nysipm.cornell.edu/factsheets/),
and in the Fruit Gardener’s Bible by
Lewis Hill and Leonard Perry. If using sprays, make sure
to read and follow
all label directions to protect yourself, the environment, and to
ensure proper
control of pests and diseases.